Many installed pipelines may be inspected using the Magnetic Flux Leakage (MFL) technique, primarily for the purpose of identifying metal loss anomalies. Magnetic flux leakage has been shown to respond in predictable ways to anomalies in the wall of the pipeline as the principal axis of the metal loss anomaly and field angle are varied. Both experimental and modeling results have been used to confirm this effect, which is also widely described in the literature.
Due in part to limitations imposed by data acquisition, data storage and magnetic circuit designs, most in-line inspection tools have employed axially oriented magnetizers (see e.g., U.S. Pat. No. 6,820,653 to Schempf et al.). However, the present axial field magnetizer designs make identification and quantification of extremely narrow axial features difficult, or in some cases, impossible. For these feature classes, a solution using a magnetic field in the circumferential or transverse direction, have been marketed and placed in service over the past decade by pipeline inspection providers. However, due to the constraints of physics, the performance and accuracy of these transverse magnetic flux inspection (TFI) tools in general is less than that of axial field tools for general metal loss anomalies.
Additionally, these TFI tools typically require a minimum of two magnetizer assemblies in order to achieve adequate coverage, making it impractical or difficult to incorporate these into an existing axial MFL tool.
For those pipelines that may have extremely narrow metal loss features, or certain classes of seam weld anomalies, standard axial field tools do not provide adequate detection and quantification capabilities. In these cases, for MFL based tools, either the initial or supplemental surveys are performed using a TFI tool. While TFI tools may be capable of detecting extremely narrow anomalies and certain seam weld anomalies, they also detect all of the remaining volumetric metal loss features typically found in pipelines, complicating the process of identifying the targeted anomaly classes.
One of the earliest TFI arrangements is described in U.S. Pat. No. 3,483,466 to Crouch et al. Crouch discloses a pair of electromagnets arranged perpendicular to each other with detectors such as magnetometers or search coils positioned on each side of the magnets. Other than the use of permanent magnets and hall device-type sensors, Crouch's arrangement remains as the basis for most modern implementations. Additionally, some designs involve segmented or individual discrete magnets that, in most cases, retain the transverse or circumferential field direction. For example, U.S. Pat. No. 3,786,684 to Wiers et al. discloses individual magnets arranged in arrays oblique to the pipe axis with the fields of each array perpendicular the others. However, this arrangement limits the field to sections and areas between the poles of each individual magnet. Furthermore, the short pole spacing required for a Wiers-type implementation decreases the length of the magnetic circuit, thereby causing the tool to suffer from velocity effects, and also masks, distorts or degrades data quality at welds, dents, or other anomalies.
Other designs involve elaborate complex geometries, multiple magnetizer sections, and elaborate mechanical arrangements such as helical drives, gears and wheels designed to induce spiral or helical motion of the magnetizer section. For example, U.S. Pat. No. 5,565,633 to Wernicke discloses a mechanically complicated device for use with magnetizer sections having two or more magnetic circuits and a plethora of sensing units. In one embodiment, the magnet blocks are arranged with spirally situated parallel poles. In another embodiment, the magnet blocks are twisted pole pairs displaced axially. Both embodiments require mechanically induced rotation in order to achieve full coverage of the inner pipe surface. Similar to Wernicke, U.S. Pat. No. 6,100,684 to Ramuat discloses a substantially transverse field magnetization arrangement that involves multiple magnetizer sections and a complex arrangement of wheels to induce helical motion of the sections and achieve overlapping or full coverage of the pipe wall. U.S. Pat. No. 7,548,059 to Thompson et al. includes two skids (poles) that incorporate fixed magnets arranged in closely spaced pairs to create a nominally transverse field spiraling around the pipe. This tool—which includes a variety of moving parts such as supporting tendons, pulleys, and springs—requires much added complexity in order to be flexible enough to accommodate bends in the pipeline. Furthermore, the magnets in this arrangement induce a field between two parallel poles, forming a single closed loop circuit between the poles of the individual discrete magnet blocks.
Similar to Thompson et al., the magnets used in the prior art are described as being blocks, with no reference to a supple or conformable upper surface used for the magnet block. Use of a rigid contact arrangement for the magnetic circuit degrades data quality by introducing air gaps or variable reluctance zones in the magnetic field path at dents or along welds and other upsets that may be present within the pipeline. For certain classes of features, disturbances created in the ambient field mask or otherwise distort the flux leakage signals present because of the features of interest. Any magnetic anomalies existing within dents and weld zones are of greater significance due to their presence within these zones and, as such, represent areas in which data quality is critical.
Additionally, the prior art requires the use of a large number of poles or surfaces in an intimate contact arrangement to the pipe wall surface. This arrangement can result in extremely high frictional forces or resistance to motion being experienced by the magnetizer assembly, thereby inhibiting or preventing its use in applications requiring lower friction.
There is a need for a MFL tool that provides full coverage of the inner pipe wall surface without the need for mechanically complicated structures; produces a field that detects axially-oriented, circumferentially-oriented and volumetric features; generates similar responses from features regardless of whether the features are axially or circumferentially oriented; eliminates or reduces velocity effects as well as signal masking, disruptions and distortion at welds, dents and other upsets; navigates pipeline obstructions, bends and reductions; and allows pipeline surveys to be accomplished in a single pass.